This invention relates to a focus detecting device for a single-lens reflex camera.
In general, in a single-lens reflex camera with a detachably mounted photographing lens, the accuracy of focusing depends on the photographer's ability to visually, accurately determine an in-focus condition, and on the environmental conditions such as brightness of the object to be photographed. Accordingly, with a reflex camera of this type, it is considerably difficult for beginners, or persons not familiar with the operation of the camera to precisely focus the camera on the object. Focusing is especially difficult in the case when a dark object is photographed, or the lens is stopped down, or in the case when an object is moving at a sufficiently high speed so that the photographer cannot determine whether or not the object's image is brought into focus.
A number of focus detecting devices utilizing electro optical effects have been proposed recently to overcome such difficulties. However, most of the focus detecting devices suffer from the following disadvantages. They are too large and too intricate and in view of their principle of operation they are liable to give a result different from a decision made by the photographer.